most, but not all ,cargo areas in passenger planes are pressurized to same as cabin - 7,000-8,000 ft. This effective altitude will be lowered (hence higher pressure) in the new Boeing 787 just released. The Boeing medical dept has felt that some travelers complaints are actually high altitude sickness.
here is one carriers info - more than you might ever need to know about pressure, humidity and temperature variation in various areas of the plane - applicable to what you send either inanimate or animate:
http://www.jal.co.jp/en/jalcargo/inter/guide/animal/http://www.jal.co.jp/en/jalcargo/inter/bulk.html
